For more than fifty years the fundamental principles of chromium plating have been those established by Colin G. Fink in U.S. Pat. No. 1,581,188 issued Apr. 20, 1926. Fink taught that the totality of all acid radicals (catalyst anions) in chromic acid chromium plating solutions had to be determined and controlled at a specific amount relative to the concentration of chromic acid in the solution. Specifically, Fink taught that in the case of sulfate the totality of all radicals should approximate 2.5 grams per liter in a solution containing 250 grams per liter of chromic acid. That is, the ratio of the chromic acid to the radicals should be approximately 100:1.
Fink discusses both stable and unstable radicals at some length without giving specific directions for the use of any of the radicals except sulfate. Fink states that it is a simple matter to ascertain the amount of any radical which is equivalent to a given quantity of sulfate radical, but this is not correct and the industry has labored under a great handicap because of the lack of specific directions for the manner in which radicals other than sulfate must be handled.
Specifically, Fink refers to four radicals as stable: sulfate, fluoride, phosphate and borate. However, phosphate and borate have essentially no catalytic effect and are not used. Fluoride is sometimes used but with great difficulty due to its instability. On a weight basis it is about four times as strong as sulfate. It is extremely reactive and forms complex salts of lesser catalytic effect with compounds such as silica, silicate, borate, aluminate, titanate, zirconate, ferrates, phosphates, antimonates, stannates, niobates, etc. A further source of the instability of fluoride is the loss of fluoride due to volatility as hydrofluoric acid.
Fink mentions nitrate and organic radicals as examples of unstable radicals but these are generally regarded as impurities in the bath and their presence is usually avoided. The same is true of the chloride which Fink regarded as an undesirable impurity.
Some promising indications of beneficial results from chloride additions to hexavalent chromium plating solutions were reported about 50 years ago but these were not followed up and chloride is generally regarded as an injurious impurity. See, for example, Dubpernell, G. "Electrodeposition of Chromium from Chromic Acid Solutions" 1977, Pergamon Press New York, pages 25-35.
A few years ago, Perakh et al suggested the use of chloride in chromium plating solutions and this work is described in U.S. Pat. No. 4,234,396 issued Nov. 18, 1980. There are, however, substantial drawbacks to the bath and process described by Perakh et al. Examples 1-6 of the Perakh et al patent refer to chromispel-C plating baths where large amounts of chromic acid and chlorides are utilized, ostensibly to increase the current efficiency. However these examples yield dull, poorly adhering deposits and the temperature range is only about 19.degree. C.-45.degree. C. which is generally considered room temperature or only slightly thereabove. In these examples it appears that the chloride is being used as the catalyst in place of the conventional sulfate radical.
Examples 16 through 19 of the Perakh patent describe the use of the plating bath including the sulfate catalyst; example 16 includes chlorine, example 17 iodine, examples 18 and 19 both chlorine and iodine. In each case, however, the deposit was dull even though the current efficiency increased in those examples where large amounts of chlorine were present.
None of the prior art, however, describes a chromium plating process wherein a good distribution of a bright, highly adherent and durable chromium plate may be obtained in the presence of both the sulfate catalyst and chlorine.